Endless chain type trenchers heretofore utilized comprised an elongated frame having one end thereof pivotally mounted on a vehicle for movement in a vertical plane parallel to the path of travel of the vehicle. The digging frame provides a mounting for a power-driven endless chain carrying a plurality of spaced digging scoops. Hydraulic cylinders are provided to pivot the frame relative to the vehicle. The depth of the trench to be dug is determined by the angular position of the digging frame with respect to the vehicle. It has been the practice to mount a shoe element to the trailing end of the digging frame which shoe performs two functions. It has an elongated horizontal bottom surface which rides along the bottom of the newly formed trench and provides a support to the free end of the trencher to maintain the digging unit in a constant depth. Variations of the angular position of said shoe relative to the digging frame may be effected by an hydraulic cylinder and thus the effective depth of the trench may be varied. Additionally, the trailing shoe may be provided with an arcuately curved passage having a horizontally disposed opening at the rear end of the shoe and a upwardly disposed opening in the forward portion of the shoe through which an endless flexible pipe or cable may be fed to lie in position in the bottom of the trench.
Where the vehicle is moving over reasonably level ground and the composition of the soil is uniform, the above described arrangement will maintain the depth of the trench being dug at a reasonably consistent level without the application of any controls. The mass of the trencher and trailing shoe in combination is sufficiently great that the trencher tends to remain at the position where the horizontal bottom of the shoe is aligned with and supported by the bottom of the trench and hence, once the desired depth is reached, the power cylinders for pivoting the digging frame relative to the vehicle are generally placed in a hydraulic floating condition so that they do not exert any forces on the digging frame except when it is desired to lift the entire frame out of the trench.
There are trenching applications, however, where a high degree of accuracy of the trench depth must be maintained irrespective of the fact that the surface terrain varies considerably in elevation and the soil conditions are not uniform.